As a typical active matrix substrate, a thin film transistor substrate (hereinafter also referred to as a “TFT substrate”) is widely used which includes thin film transistors (hereinafter also referred to as “TFTs”) as switching elements, one for each pixel, which is the smallest unit of an image.
A TFT having a typical configuration includes, for example, a gate electrode provided on an insulating substrate, a gate insulating film covering the gate electrode, an island-like semiconductor layer provided on the gate insulating layer over the gate electrode, and a source electrode and a drain electrode provided on the semiconductor layer, facing each other.
The TFT substrate includes a plurality of TFTs provided on a substrate and arranged in a matrix, an insulating film covering the TFTs and having a flat surface, and a plurality of pixel electrodes provided on the insulating film, each corresponding to one of the TFTs. In a liquid crystal display panel, a counter substrate is disposed to face the TFT substrate, and a liquid crystal layer is provided between the TFT substrate and the counter substrate.
In recent years, for the TFT substrate, a TFT including a semiconductor layer of an oxide semiconductor (hereinafter referred to as an oxide semiconductor layer) instead of a semiconductor layer of amorphous silicon (a-Si) has been proposed as a TFT for each pixel, which is the smallest unit of an image. The oxide semiconductor layer is more excellent than the a-Si semiconductor layer in the following respects: better switching characteristics; lower off current value; higher electron mobility; availability of a low-temperature process; etc. In particular, because of the better switching characteristics and the lower off current value, the layout of the TFTs can be caused to be more compact, leading to an increase in aperture ratio. Therefore, a reduction in power consumption and an increase in resolution are expected.
Incidentally, the electrical characteristics of oxide semiconductors are known to depend significantly on the concentration of oxygen. Therefore, for example, when the concentration of oxygen around the oxide semiconductor layer varies due to a thermal treatment, the electrical characteristics of the oxide semiconductor layer disadvantageously degrade.
PATENT DOCUMENT 1 describes an oxide semiconductor TFT which includes a channel protection layer provided above the gate electrode, covering at least a portion of side surfaces of the semiconductor layer exposed from the source and drain electrodes. Here, the source and drain electrodes have, for example, a multilayer structure including a Mo film, an Al film, and a Mo film. PATENT DOCUMENT 1 states that because the channel protection layer is thus provided, even if a thermal treatment for forming a passivation film etc. is performed, a change in the concentration of oxygen contained in the semiconductor layer is reduced or prevented, and therefore, a degradation in electrical characteristics is reduced or prevented. PATENT DOCUMENT 1 also states that if a thermal treatment is performed at as high as 350° C. or more in order to restore the degraded electrical characteristics of the semiconductor layer, the semiconductor layer reacts with the source and drain electrodes, so that hillocks (projections and recesses) disadvantageously occur on the source and drain electrodes, and that even when the thermal treatment for forming a passivation film etc. is performed, the degradation in the electrical characteristics of the semiconductor layer is reduced or prevented, and therefore, the thermal treatment at 350° C. is not performed after the formation of the source and drain electrodes.